An amplifier can convert an input voltage signal to an output current signal, i.e., the amplifier can act as a transconductance and can be called a transconductance amplifier. The transconductance is the ratio of the output current signal to the input voltage signal.
The transconductance amplifier's transconductance, frequency response, impedance, noise, bias requirements, and many other amplifier characteristics are functions of the transistor semiconductor technology used. The semiconductor technologies that can be used for the transconductance amplifier can include bipolar junction transistor (BJT), junction field effect transistor (JFET), metal oxide semiconductor field effect transistor (MOSFET) technology, or complementary MOSFET (CMOS) technology.
A differential-input, differential-output transconductance amplifier can contain a differential pair of transistors. The voltage signal can be applied to the gates of two MOSFET transistors in the differential pair. Each transistor of the differential pair can generate a separate output current that can be nearly equal in magnitude and opposite in direction. One output current can flow into one transistor of the differential pair and another output current can flow out of the corresponding paired transistor. In other words, the two output currents can flow in antiphase, i.e., 180° out of phase, and can be regarded as a differential current.
For MOSFET technology, the source terminal of each transistor in the differential pair can be directly connected, thereby forming a source-coupled pair. Each corresponding drain terminal can provide an output current. For simplicity, MOSFET gate, source, and drain terminals are discussed in this disclosure rather than the equally applicable BJT base, emitter, and collector terminals, respectively. It is to be understood an input current signal into the base of the BJT develops an input voltage signal due to the input impedance at the base.
The two source terminals in the source-coupled pair can share a current sink that biases the pair. The current sink can be divided into two smaller, nearly identical current sinks each carrying half of the bias current of the single current sink. The two smaller current sinks can be connected in parallel so the two half currents add equally to the bias current obtained from a single current sink.
The gain and frequency response of a source-coupled pair can be modified by adding one or more degenerating impedances. A degenerating impedance can decrease the degenerated gain of an amplifier, such as a transconductance amplifier, by using a portion of the undegenerated gain for negative feedback. A degenerating impedance can improve the linearity of an amplifier and can impart frequency-selective characteristics to the transfer function of the amplifier. Frequency-selective characteristics may be described in terms of bands, which are ranges of a frequency variable.
Degenerating impedances can increase the range of input voltage over which the differential amplifier operates linearly. When two degenerating impedances are used to degenerate a differential pair of MOSFET transistors, each source terminal in the differential pair can drive a terminal of a degenerating impedance that is interposed between the source terminal and a single, common current sink.
A single degenerating impedance can be connected to the differential pair instead of two degenerating impedances. The single degenerating impedance can bridge or cross-couple the two source terminals. The common current sink can be replaced by two separate current sinks when a single degenerating impedance is used.
Each of the separate current sinks can connect to a separate source terminal of the differential pair. The separate current sinks can bias each transistor of the differential pair by drawing equal bias currents from the source terminals of each transistor. The bias currents can situate the two transistors in a linear range of operation without a DC voltage drop across the single degenerating impedances, which may then modify the transconductance gain and transconductance spectral shape of the differential amplifier.